Points FOR Established Settings

Common ground. If you use one of the big settings such as Forgotten Realms, Greyhawk, Eberron, Ravenloft, Chicago from The Dresden Files, the odds are good you will have at least one other player who has either heard of them, read fiction set in the world, or even played or GMed adventures there. Having that background can really let you focus on other aspects of the game such as stories and character-driven plots.

Plenty of existing adventures to choose from. Many of the big shared worlds have been in existence for decades, meaning that there’s a good chance you’ll find an adventure you can either run as is or tweaked slightly for a group. And even if you don’t run them, there are plenty of ideas to harvest for your own stories.

Easy to find inspiration in novel form. Depending on which publisher, RPG setting-based novels are quite popular and have been for years. I have read many of the Shadowrun novels, BattleTech, and Dragonlance books over the years as well as all of The Dresden Files and similar urban fantasy series. It’s easy to lift plots, NPCs, and more from these sources.

Historically-accurate settings can be great sources of inspiration. We live and breathe in a world with a ton of history. Imagine playing a 1920s or 30s era supernatural investigator for Call of Cthulhu and think of all the great places and historical events you could work into your alternate version for stories. Real life is a huge established setting if you look at it in the right way, whether just for window-dressing or for more immersive worlds.

Points AGAINST Established Settings

Common ground. If you use one of the big settings such as Forgotten Realms, Greyhawk, etc., the odds are also good you might find a know-it-all who will nitpick anything you choose to do as a GM, especially if you change the canon in some way.

Plenty of existing adventures to choose from. If you choose to run an existing module, you may find someone who has gone through it as a player or run it as a GM. If any meta-gaming creeps into a session, it can quickly go from fun to flat for the GM or players quickly.

Any story elements brought from setting-based novels have a chance of being recognized and nitpicked by the same folks who nitpick settings and adventures.

Any historically- or real-world-based setting has a chance of being picked apart by players who have read widely or experienced the world firsthand. It’s tough to compete with an actual historian refuting the events of the “Roaring ’20s” or a wizened traveler who has visited medieval ruins saying “that’s just not the way that looks/works/feels…”

In Conclusion…

I could go on and on and on… But I won’t. Why? Because any reason you have for playing in an established setting is a good one so long as your group is having fun with it. You’re not going to reenact the events of a novel, film, or history exactly at the table anyway. There, the goal is to enjoy yourself with
friends in two worlds – the real one and an imagined one. Just let those nitpicky things go and have a good time!

If you’re having fun, you must be doing it right – whatever it is – so party on people and forget the naysayers!